Wavelength division multiplex optical communications systems are of growing importance in the transmission of large quantities of information, such as the transmission of images. Integrated lasers producing at least two light beams of different wavelengths are particularly useful in multiplexed transmission of information in optical communications systems. In those systems, an integrated laser producing at least two different wavelength laser beams simplifies optical alignments and optical matching.
Semiconductor lasers including two or more active regions, each active region producing light of a different wavelength, are known. However, the lasers require complicated manufacturing processes that result in poor production yields. An example of such an integrated semiconductor laser is described in Japanese published patent application 62-48917. The laser structure described in that publication is shown in a schematic perspective view in FIG. 7. A method of manufacturing that laser structure is shown in FIGS. 8(a)-8(d).
The laser of FIG. 7 includes three active regions disposed on a base including an n-type indium phosphide substrate 21 and an n-type indium phosphide buffer layer 22 disposed on the substrate. In a first active region, at the left of the structure as shown in FIG. 7, a first indium gallium arsenide phosphide active layer 31 is disposed directly on the buffer layer 22. The relative proportions of the constituents of the indium gallium arsenide phosphide active layer 31 are adjusted for laser oscillation at a first wavelength. In a central active region, layer 31 is also present and successively disposed on it are a first intermediate n-type indium phosphide layer 41 and a second indium gallium arsenide phosphide active layer 32. The relative proportions of the constituents of the second active layer 32 are adjusted for laser oscillation at a second wavelength different from the wavelength at which laser oscillations are produced in the first active layer 31. In a third active region, shown at the right side of FIG. 7, the same layers are present as in the central active region. In addition, a second n-type indium phosphide intermediate layer 42 and a third indium gallium arsenide phosphide active layer 33 are successively disposed on the first intermediate layer 41. The relative proportions of the constituents of the third indium gallium arsenide phosphide active layer 33 are adjusted for laser oscillation at a third wavelength different from the wavelengths produced in the first and central active regions. Each of the active regions is confined at its sides by a p-type indium phosphide current blocking layer 23 and an n-type indium phosphide current confining layer 24 disposed on layer 23. The active regions and the current confining layer 24 are covered by a p-type indium phosphide layer 25. Finally, an n-type indium gallium arsenide phosphide contact layer 26 is disposed on the p-type indium phosphide layer 25. A silicon dioxide film 27 is selectively disposed on the contact layer 26 and includes openings opposite each of the three active regions. Electrodes 51, 52, and 53 are disposed on the silicon dioxide film 27 and in contact with the contact layer 26 respectively opposite each of the active regions. To improve the quality of the contact, zinc is diffused through layer 26 and into layer 25 at regions 71, 72, and 73 respectively opposite the first, central, and third active regions. The electrodes 51, 52, and 53 respectively contact regions 71, 72, and 73. A common electrode 61 is disposed on the substrate opposite the buffer layer 22.
The process for manufacturing the laser structure of FIG. 7 is relatively complex. Steps in that process are illustrated in FIGS. 8(a)-8(d). As shown in FIG. 8(a), the buffer layer 22, the first active layer 31, the first intermediate layer 41, the second active layer 32, the second intermediate layer 42, and the third active layer 33 are successively grown on substrate 21. As illustrated in FIG. 8(b), the grown films are selectively etched to expose active layers 31, 32, and 33 over respective lengths of about two hundred microns. The active regions of the laser are then prepared by etching ridges 81, 82, and 83 lying along the &lt;110&gt; direction and having a width of two to three microns as illustrated in FIG. 8(c). The ridges are defined by respective etching masks 91, 92, and 93. Subsequently, as illustrated in FIG. 8(d), the current blocking layer 23 and current confining layer 24 are successively grown adjacent the sides of the ridges. Finally, the p-type indium phosphide layer 25 and the n-type contact layer 26 are successively grown on the current confining layer 24 and each of the ridges. Thereafter, diffusion masks, such as the layer 27, are deposited on the contact layer. The masks each include an opening disposed opposite the ridges 81, 82, and 83, typically of a width of about ten microns. Zinc is diffused through the openings in the diffusion masks to a depth to reach the p-type layer 25 and establish contact to the respective uppermost active layers at each ridge. Electrodes 51, 52, and 53 are deposited on the diffusion masks in contact with the respective zinc-diffused regions 71, 72, and 73. A common electrode 61 is deposited on the reverse side of the substrate.
In the resulting structure, each of the active regions can be separately forward biased and each oscillates at a different wavelength, providing three light beams that can be independently generated and modulated to increase the amount of information transmitted in an optical communications system. However, the method of manufacturing the integrated laser is so complicated that it is difficult to manufacture the structure economically.
Another integrated semiconductor laser structure having three active regions and a single substrate is shown in a perspective, partially cut-away view in FIG. 9. The active regions 101, 102, and 103 interact with respective diffraction gratings 201, 202, and 203. The periods of the respective diffraction gratings are different in order to produce different wavelength light at each of the active regions. The diffraction gratings are produced by a conventional technique in which interference fringes illuminate a resist film before its development and subsequent etching. However, it is difficult to control the different periods of the three gratings and each of the laser sections includes a wavelength control adjustment portion 301, 302, and 303, respectively, for tuning the oscillation wavelengths. An insulating film 400 separates the respective electrodes 501, 502, and 503 from the substrate. A common electrode 600 is disposed on the opposite side of the substrate. The three active regions, i.e., laser elements, are mutually isolated by grooves 701 and 702.
Like the structure of FIG. 7, the complex structure of FIG. 9 requires many complicated processing steps, particularly in the formation of several diffraction gratings, each having a different period. As a result, the production yield is very poor, resulting in high costs.
Accordingly, it would be desirable to produce an integrated laser including at least two active regions, each active region producing laser light at a different wavelength in a relatively simple process providing good product yield at reasonable product cost.